This invention relates generally to fluid impellers and more particularly to cavitation resistant fluid impellers made from castable cavitation resistant austenitic chromium-manganese alloy steels.
Pump impellers frequently suffer cavitation damage for several reasons, including operation outside established hydraulic parameters. This damage is often a limiting factor in the life of the equipment. It may not be repairable by welding for reasons of inaccessibility. With a growing emphasis on enhanced reliability and longer life, there is a need in the pump industry for a casting alloy with significantly better cavitation resistance than the standard materials used to manufacture impellers. Other characteristics required for such a material to be commercially viable include machinability and weldability.
For high speed applications, relatively high tensile and yield strength, and elongation will also be necessary. The mechanical properties of commonly used austenitic stainless steels, such as CF8M are: tensile strength 482 N/mm.sup.2 and yield strength 208 N/mm.sup.2 minimum. These low mechanical properties render such materials unsuitable for high speed impellers.
The current state-of-the-art cavitation resistant material which has been used in pumps is a cobalt modified austenitic stainless steel known as Hydroloy.RTM.. Hydroloy.RTM. is described in U.S. Pat. No. 4,588,440, Co Containing Austenitic Stainless Steel with High Cavitation Erosion Resistance. One deficiency of Hydroloy.RTM. is susceptibility to hot short cracking. This characteristic contributes to poor castability. The presence of cobalt is also undesirable for some applications, particularly the nuclear industry.
The foregoing illustrates limitations known to exist in present cavitation resistant alloy steels. Thus, it is apparent that it would be advantageous to provide an alternative directed to overcoming one or more of the limitations set forth above. Accordingly, a suitable alternative is provided including features more fully disclosed hereinafter.